Piezoelectric actuator, ink-jet head provided with the same, ink-jet printer, and method for manufacturing piezoelectric actuator

ABSTRACT

A piezoelectric actuator  3  includes a metallic vibration plate  30 , an insulating layer  31 , a plurality of individual electrodes  32 , a piezoelectric layer  33  and a common electrode  34 . The insulating layer  31  is formed on the top surface of the vibration plate  30 . The individual electrodes  32  are formed on the top surface of the insulating layer  31 . The piezoelectric layer  33  is formed on the top surfaces of the individual electrodes  32 . The common electrode  34  is formed on the top surface of the piezoelectric layer  33  over the individual electrodes  32 . A plurality of terminals  36  and a plurality of wirings  35  are formed on the top surface of the insulating layer  31 . Each of the terminals  36  is associated with one of the individual electrodes  32 . Each of the wirings  35  connects one of the individual electrodes  32  and the associated terminal  36.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric actuator for use in anink-jet head or the like, an ink-jet head provided with thispiezoelectric actuator, an ink-jet printer, and a method formanufacturing a piezoelectric actuator.

2. Description of the Related Art

Piezoelectric actuators are conventionally known, which drive drivenobjects by deforming piezoelectrics. For example, this kind ofpiezoelectric actuator is employed in an ink-jet head such as the onedescribed in Japanese Patent Application Laid-open No. 2003-154646. Thispiezoelectric actuator has a piezoelectric made of lead zirconatetitanate (PZT), lower electrodes and upper electrodes. The lowerelectrodes are formed on the bottom surface of the piezoelectric, andeach of them is associated with one of the pressure chambers in theink-jet head. The upper electrodes are formed on the top surface of thepiezoelectric, and each of them is associated with one of the lowerelectrodes. A wiring extends from each of the lower and upper electrodesand has an electric contact fitted on its one end. A driver IC (a driveunit) is connected to the electric contacts of the wirings for the lowerand upper electrodes and supplies drive voltage to the piezoelectricactuator. When drive voltage is supplied to the piezoelectric actuator,the potential of the lower electrodes differs from that of the upperelectrodes. This results in an electric field acting on thepiezoelectric, which is sandwiched between the electrodes. The fieldaction deforms the piezoelectric, thereby applying pressure on the inkin the pressure chambers.

However, in the piezoelectric actuator of Japanese Patent ApplicationLaid-open No. 2003-154646, the electric contacts for the respectivelower electrodes and the electric contacts for the respective upperelectrodes are formed at different heights. Accordingly, it is difficultto directly connect the electric contacts, which are formed at differentheights, and the output terminals of the driver IC. This makes itnecessary to use a flexible wiring member, such as a flexible printedwiring board (or FPC: flexible printed circuit). However, such a wiringmember is generally expensive.

In recent years, there have been needs for higher resolutions in printquality and more compact ink-jet heads. These needs may be met by densearrangement of the electrodes and electric contacts of the piezoelectricactuator. However, the dense arrangement makes it necessary to use awiring member having a narrower wiring pitch. Because such a wiringmember is more expensive, the cost of manufacturing the piezoelectricactuator is higher.

The electric contacts positioned at different heights and the driver ICmay be connected by one wiring member. In this case, the wiring memberbends when the electric contacts, which are positioned at differentheights, and the connecting terminals of the wiring member areconnected. This may decrease the reliability of the electric connection.Alternatively, each of the electric contacts positioned at differentheights and the driver IC may be connected by a wiring member. Thisincreases the number of wiring members, thereby increasing the cost ofelectrically connecting the electric contacts and the driver IC.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a piezoelectricactuator which ensures high reliability of the electric connection witha drive unit and reduces the cost of the electric connection.

Another object of the invention is to provide a method of manufacturingsuch a piezoelectric actuator.

Still another object of the invention is to provide an ink-jet headhaving such a piezoelectric actuator.

A further object of the invention is to provide an ink-jet printerprovided with such an ink-jet head.

According to a first aspect of the present invention, there is provideda piezoelectric actuator comprising:

-   -   a metallic vibration plate;    -   an insulating layer formed on a surface of the vibration plate;    -   a plurality of individual electrodes formed on a surface of the        insulating layer;    -   a piezoelectric layer formed on surfaces of the individual        electrodes;    -   a common electrode formed on a surface of the piezoelectric        layer and common to the individual electrodes;    -   a plurality of first terminals formed on the surface of the        insulating layer and each associated with one of the individual        electrodes; and    -   a plurality of first wiring formed on the surface of the        insulating layer and each electrically connecting one of the        individual electrodes to one of the first terminals.

A first characteristic of the present invention is that the plurality ofindividual electrodes, the plurality of first terminals and theplurality of first wirings are formed on the surface of the insulatinglayer, which is formed on the vibration plate. Each of the firstterminals is associated with one of the individual electrodes. Each ofthe first wirings electrically connects one of the individual electrodesto the associated first terminal. Thus, the individual electrodes, thefirst terminals therefor and the first wirings connecting theseelectrodes and terminals are formed on the same plane. This makes itpossible to directly connect the individual electrodes and the firstterminals, to which a drive unit for driving the piezoelectric actuatoris connected, by the first wirings which are formed on the same plane asthese electrodes and terminals, without using a costly wiring membersuch as an FPC. As a result, the cost of electric connection becomeslower, and the reliability of electric connection becomes higher.

A second characteristic of the present invention is that the commonelectrode, which is common to the individual electrodes, is arranged onthe top surface of the piezoelectric layer. More specifically, anelectrode is not formed for each of the individual electrodes tocooperate with it in generating an electric field across thepiezoelectric layer, but the single common electrode, which is common toall or the groups of the individual electrodes, is formed on thepiezoelectric layer. Accordingly, the number of terminals for connectingthe common electrode to the drive unit or the like is very small incomparison with the terminals connecting the individual electrodes,which are formed on the insulating layer, to the drive unit or the like.For example, when the common electrode is formed over all of theindividual electrodes, only one terminal is needed for the commonelectrode. Consequently, there is no need to use a wiring member toconnect the common electrode, for which a fewer terminals are needed, tothe drive unit or the like. The small number of terminals leads to easyand reliable electric connection. In addition, because the vibrationplate is metallic and highly elastic, the piezoelectric actuator ishighly responsive.

Even though pairs of the electrodes for applying electric fields to thepiezoelectric layer are formed at different heights, the first andsecond characteristics eliminate the need for using an expensive wiringmember such as an FPC to connect the terminals for the respective pairsof electrodes to the drive unit.

In the piezoelectric actuator of the present invention, the individualelectrodes is arranged on the surface of the insulating layer, whichelectrically insulates adjoining individual electrodes from each other.Drive voltage is applied selectively to the individual electrodes. Whenthe common electrode, arranged on the top surface of the piezoelectriclayer, differs in potential from the individual electrodes to which thedrive voltage is applied and which is arranged on the bottom surface ofthe piezoelectric layer. Then, the potential difference generates anelectric field across the portion of the piezoelectric layer that issandwiched between the common electrode and each of these individualelectrodes. The electric field deforms the piezoelectric layer. Thedeformation of the piezoelectric layer deforms the vibration plate,there by driving a driven object.

In the piezoelectric actuator of the present invention, a secondterminal which is associated with the common electrode and a secondwiring which electrically connects the common electrode and the secondterminal may be formed on the surface of the insulating layer. In suchan actuator, the plurality of first terminals for the respectiveindividual electrodes and the second terminal for the common electrodeare formed on the surface of the insulating layer. Accordingly, all ofthe first and second terminals can be positioned at the same height.This makes it easy to join the output terminals of the drive unit whichdrives the piezoelectric actuator, and the first and second terminals.This also increases the reliability of the electric connection of thejoined parts. When forming all of these terminals on the surface of theinsulating layer, it is necessary to merely form the first and secondwirings on this surface. This makes it possible to position theterminals at the same height with a simple wiring structure that doesnot require the use of a complex structure, such as through holes. Inaddition, the common electrode can be grounded.

In the piezoelectric actuator of the present invention, the insulatinglayer may be formed of a ceramic material. When the insulating layer isformed of a ceramic material, which has a high coefficient ofelasticity, the actuator is more rigid and more responsive.

The piezoelectric actuator of the present invention may further includea drive unit which supplies drive voltage selectively to the individualelectrodes, the drive unit being placed on the surface of the insulatinglayer, the drive unit having a plurality of output terminals joined tothe first terminals and the second terminal. Accordingly, the individualelectrodes and the common electrode are connected to the drive unitdirectly by the first and second wirings. This eliminates the need for awiring member such as an FPC and reduces the cost of electricconnection.

In the piezoelectric actuator of the present invetnion, the drive unitmay further have an input terminal which is joined to a connectingterminal for electrically connecting the drive unit and a control unitwhich controls the drive unit. The connecting terminal may be formed onthe surface of the insulating layer. In general, a drive unit isconnected to a control unit for controlling it. According to the presentinvention, a connecting terminal which connects the control unit and thedrive unit with an FPC, a flexible flat cable (FFC) or the like can beformed at one time when the plurality of individual electrodes, theplurality of terminals and the plurality of wirings are formed.

According to a second aspect of the present invention, there is providedan ink-jet head comprising:

-   -   the piezoelectric actuator of the present invention; and    -   a channel unit having a plurality of pressure chambers and        nozzles which discharge ink, the pressure chambers each        communicating with one of the nozzles, the pressure chambers        being open on a surface of the channel unit, and pressure        chambers each being associated with one of the individual        electrodes of the piezoelectric actuator;    -   wherein the vibration plate of the piezoelectric actuator is        formed of any one of iron alloy, nickel alloy and aluminum        alloy, and laid on the surface of a portion of the channel unit;        and    -   wherein at least the portion of the channel unit on which the        vibration plate is laid is formed of any one of iron alloy,        nickel alloy and aluminum alloy.

In the ink-jet head of the present invention, because at least a part ofthe channel unit of the ink-jet head is formed of any one of iron alloy,nickel alloy and aluminum alloy, it is easy to form ink channels in thechannel unit by etching or the like. In addition, because the vibrationplate is formed of similar material, the channel unit and the vibrationplate have similar coefficients of thermal expansion, which improvetheir joining strength.

In the ink-jet head of the present invention, the vibration plate may beformed of a same material as that of at least the portion of the channelunit on which the vibration plate is laid. Accordingly, the vibrationplate and this portion of the channel unit have more similarcoefficients of thermal expansion, so that their joining strength isfurther increased. The ink in the channel unit is in contact with thechannel unit and the vibration plate that are formed of the same kind ofmaterial. As a result, no local cell is formed regardless of theselection of ink type. Thus, there are no restrictions on ink selectionswith regard to corrosion, so a user has greater freedom to select theink to use.

In the ink-jet head of the present invention, the vibration plate and atleast the portion of the channel unit on which the vibration plate islaid may be formed of stainless steel. This improves the corrosionresistance of both the vibration plate and this portion of the channelunit against ink, so a user has still greater freedom to select the inkto use.

According to a third aspect of the present invention, there is provideda method of manufacturing a piezoelectric actuator, the methodcomprising:

-   -   a first step of forming an insulating layer on a surface of a        metallic vibration plate;    -   a second step of forming a plurality of individual electrodes,        first terminals and first wirings on a surface of the insulating        layer, the first terminals each being associated with one of the        individual electrodes, the first wirings each electrically        connecting one of the individual electrodes and one of the first        terminals;    -   a third step of forming a piezoelectric layer on surfaces of the        individual electrodes; and    -   a fourth step of forming a common electrode on a surface of the        piezoelectric layer over the individual electrodes.

According to this method, it is possible to easily manufacture apiezoelectric actuator with which effects can be achieved which aresimilar to those of the first aspect of the present invention. Inaddition, in the second step, the individual electrodes, the firstterminals and the first wirings can be formed at a time on the surfaceof the insulating layer. This makes it possible to shorten themanufacturing process.

In the method of the present invention, the fourth step may includeforming a second terminal and a second wiring together with the commonelectrode on the surface of the insulating layer, the second terminalbeing associated with the common electrode, the second wiringelectrically connecting the common electrode and the second terminal.Accordingly, it is possible to easily manufacture the above-describedpiezoelectric actuator. In addition, in the fourth step, the commonelectrode and the second terminal and the second wiring corresponding tothe common electrode, respectively, can be formed at a time on thesurface of the insulating layer. This makes it possible to shorten themanufacturing process.

The method of the present invention may further comprise a fifth stepof:

-   -   placing, on the surface of the insulating layer, a drive unit        which supplies drive voltage selectively to the individual        electrodes, and has output terminals; and    -   joining the output terminals to the first terminals and the        second terminal. In this case, the individual electrodes and the        drive unit can be connected directly with the first and second        wirings. This eliminates the need for a wiring member such as an        FPC and reduces the cost of electric connection.

In the first step, the insulating layer may be formed on the surface ofthe vibration plate by an aerosol deposition method. With this method,it is possible to form a film having a sufficient insulation propertywith a thickness of not more than 5 μm. In particular, it is possible toform a film which is excellent in insulation property even with athickness of not more than 2 μm.

In the method of the present invention, the second step may also includeforming, on the surface of the insulating layer, a connecting terminalwhich electrically connects the drive unit and a control unit whichcontrols the drive unit, the connecting terminal being jointed to aninput terminal of the drive unit. Accordingly, the connecting terminalfor connecting the control unit and the drive unit can be formed at thesame time that the individual electrodes, the first terminals and thefirst wirings are formed.

According to a fourth aspect of the present invention, there is provideda piezoelectric actuator which comprises:

-   -   a piezoelectric layer;    -   a vibration plate provided on one surface of the piezoelectric        layer;    -   a plurality of individual electrodes interposed between the        piezoelectric layer and the vibration plate;    -   a common electrode provided on the other surface of the        piezoelectric layer and common to the individual electrodes;    -   a plurality of first terminals formed on a same plane as the        individual electrodes and each associated with one of the        individual electrodes; and    -   a plurality of first wirings formed on the same plane as the        individual electrodes and each electrically connecting one of        the individual electrodes and one of the first terminals. In        this actuator, the plurality of individual electrodes are        arranged between the piezoelectric layer and the vibration        plate. Accordingly, it is possible to arrange the first        terminals and the first wirings for the individual electrodes on        the same plane with the individual electrodes. In addition, the        common electrode is common for the plurality of individual        electrodes, the number of the wirings and terminals for the        electrodes can be small. By adopting such a specific arrangement        for positioning the individual electrodes and by using such a        common electrode, it is possible to arrange a drive unit which        applies a driving voltage to the individual electrodes on the        actuator, without using a wiring member such as FPC.

The vibration plate may be formed of a metallic material, and aninsulating layer may be formed on the vibration plate. Alternatively,the vibration plate may be formed of an insulating material.

A second terminal associated with the common electrodes and a secondwiring electrically connecting the common electrode and the secondterminal may be formed on a same plane as the plurality of individualelectrodes. In particular, a structure may be adopted in which theplurality of individual electrodes, the first terminals, the firstwirings, the second terminal, and the second wiring are formed on asurface of the insulating layer; and in which a drive unit whichsupplies a drive voltage selectively to the individual electrodes isprovided on the surface of the insulating layer, the drive unit having aplurality of output terminals joined to the first terminals and thesecond terminal. This structure may be an alternative to a wiringstructure using a conventional wiring member such as FPC.

According to the present invention, there is provided an ink-jet headcomprising:

-   -   the piezoelectric actuator according to the fourth aspect of the        present invention; and    -   a channel unit having a plurality of pressure chambers and        nozzles which discharge ink, the pressure chambers each        communicating with one of the nozzles, the pressure chambers        being open on a surface of the channel unit, the pressure        chambers each being associated with one of the individual        electrodes of the piezoelectric actuator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an ink-jet head according to anembodiment of the present invention.

FIG. 2 is a schematic plan view of the right half of the ink-jet headshown in FIG. 1.

FIG. 3 is a sectional view taken on the line III-III of FIG. 2.

FIG. 4 is a sectional view of the line IV-IV of FIG. 2.

FIG. 5 is a view showing a step of joining a channel unit and avibration plate.

FIG. 6 is a view showing a step (first step) of forming an insulatinglayer.

FIG. 7 is a view showing a step (second step) of forming individualelectrodes.

FIG. 8 is a view showing a step (third step) of forming a piezoelectriclayer.

FIG. 9 is a view showing a step (fourth step) of forming a commonelectrode.

FIG. 10 is a view showing a step (fifth step) of placing a driver ICover the insulating layer.

FIG. 11 is a view showing a step of joining a nozzle plate.

FIG. 12 is a view showing the pattern of common electrode in a secondembodiment of the present invention.

FIG. 13 schematically shows an embodiment of an ink-jet printer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

The following is a description of an embodiment of the presentinvention. The embodiment is an example of the present invention as itis applied to a piezoelectric actuator for use in an ink-jet head.

As is shown in FIG. 1, an ink-jet head 1 includes a channel unit 2 and apiezoelectric actuator 3. The channel unit 2 has ink channels formedtherein. The piezoelectric actuator 3 is laminated to or stacked on thetop surface of the channel unit 2.

The channel unit 2 will be described below. As is shown in FIGS. 2 to 4,the channel unit 2 includes a cavity plate 10, a base plate 11, amanifold plate 12, and a nozzle plate 13. These four plates 10 through13 are joined together in stacked layers. The cavity plate 10, the baseplate 11, and the manifold plate 12 are substantially rectangular andmade of stainless steel. For that reason, it is easy to form inkchannels, such as a manifold 17 and pressure chambers 14, which will bedescribed later on, in the three plates 10 through 12 using an etchingprocess. The nozzle plate 13 may be formed of high molecular syntheticresin (such as polyimide) and is joined to the bottom surface of themanifold plate 12. Alternatively, the nozzle plate 13 may be formed ofmetallic material, such as stainless steel, like that used for the threeplates 10 through 12.

As is shown in FIG. 2, the cavity plate 10 has a plurality of pressurechambers 14 formed through it, which are arranged along a plane. Thepressure chambers 14 are open on the top surface of the channel unit 2(the top surface of the cavity plate 10, to which a vibration plate 30,described later on, is joined). FIG. 2 shows a part (eight) of thepressure chambers 14. Each of the pressure chambers 14 is substantiallyelliptic in a plan view and is arranged so that the long axis directionthereof is parallel to the longitudinal direction of the cavity plate10.

The base plate 11 has communicating holes 15 and 16 formed through itfor each of the pressure chambers 14. Each of the communicating holes 15is formed at a position which overlaps in a plan view with one end ofthe associated chamber 14 in the long axis direction thereof. Each ofthe communicating holes 16 is formed at a position which overlaps in aplan view with the other end of the associated chamber 14 in the longaxis direction thereof. The manifold plate 12 has a manifold 17 formedthrough it. Portions of the manifold 17 extend in two rows in thelateral direction of the manifold plate 12 (vertical direction in FIG.2). The manifold 17 overlaps in a plan view with the right halves inFIG. 2 of the pressure chambers 14. The cavity plate 10 has an inksupply port 18 formed through it, through which ink is supplied from anink tank (not shown) to the manifold 17. The manifold plate 12 also hascommunicating holes 19 formed through it, each of which is formed at aposition which overlaps in a plan view with the left end in FIG. 2 ofone of the pressure chambers 14. The nozzle plate 13 has a plurality ofnozzles 20 formed through it, each of which is formed at a positionwhich overlaps in a plan view with the left end of one of the pressurechambers 14. The nozzles 20 are formed by processing a substrate of highmolecular synthetic resin, such as polyimide, with an excimer laser.

As shown in FIG. 3, the manifold 17 communicates with the pressurechambers 14 via the communicating holes 15, and the pressure chambers 14communicate with the nozzles 20 via the communicating holes 16 and 19.Thus, the channel unit 2 has individual ink channels formed therein andleading from the manifold 17 through the pressure chambers 14 and to thenozzles 20 respectively.

The piezoelectric actuator 3 will be described below. As shown in FIGS.1 to 4, the piezoelectric actuator 3 includes a vibration plate 30, aninsulating layer 31, individual electrodes 32, a piezoelectric layer 33,and a common electrode 34. The vibration plate 30 is arranged on the topsurface of the channel unit 2. The insulating layer 31 is formed on thetop surface of the vibration plate 30. The individual electrodes 32 areformed on the top surface of the insulating layer 31, and each of theindividual electrodes 32 is associated with one of the pressure chambers14. The piezoelectric layer 33 is formed on the top surfaces of theindividual electrodes 32. The common electrode 34 is formed on the topsurface of the piezoelectric layer 33 over the individual electrodes 32and is common to the individual electrodes (in case of this embodiment,the common electrode 34 is common to all of the individual electrodes32).

The vibration plate 30 is a stainless steel plate that is substantiallyrectangular in a plan view. The vibration plate 30 covers the openingsof the pressure chambers 14 and is joined in a laminated state to thetop surface of the cavity plate 10. Because the vibration plate 30 isformed of stainless steel, which has a comparatively high coefficient ofelasticity, the rigidity of the vibration plate 30 makes thepiezoelectric actuator 3 highly responsive when the piezoelectric layer33 is deformed to discharge ink, as described later on. The vibrationplate 30 is joined to the top surface of the cavity plate 10, which isalso formed of stainless steel. For that reason, the vibration plate 30and cavity plate 10 have equal coefficients of thermal expansion, whichimprove the strength of their joint. The ink in the channel unit 2 is incontact with the vibration plate 30 and the channel unit 2, which areformed of stainless steel. Because stainless steel is highly resistantto ink corrosion, there is no possibility that local cells are formed inthe channel unit 2 or the vibration plate 30, regardless of theselection of ink type. Thus, there are no restrictions on ink selectionswith regard to corrosion, so a user has greater freedom to select theink to use.

The insulating layer 31 is formed on the top surface of the vibrationplate 30 and is formed of alumina, zirconia, silicon nitride or otherceramic material having a high coefficient of elasticity. The surface ofthe insulating layer 31 is flat. Because the insulating layer 31 isformed of ceramic material, which has a high coefficient of elasticity,the actuator is more rigid and more responsive. Because the insulatinglayer 31 is formed on the top surface of the vibration plate 30, theindividual electrodes 32 can be formed on the vibration plate 30, eventhough the vibration plate 30 is formed of stainless steel, which is anappropriate material, as described above, not an insulating material. Inthis application, the “insulating material” represents material that isintentionally used to inhibit the electric conduction among theindividual electrodes 32, and the “insulating layer” represents a layerformed of such material. Accordingly, for example, the “insulatinglayer” does not include a layer formed mainly of lead zirconate titanate(PZT), of which the piezoelectric layer 33 is formed.

The individual electrodes 32 are formed on the top surface of theinsulating layer 31. The individual electrodes 32 are flat, elliptic andsmaller than the pressure chambers 14 to a certain extent. Theindividual electrodes 32 are formed at positions which overlap in a planview with the central areas of the respective pressure chambers 14. Theindividual electrodes 32 are made of conductive material such as gold.Adjoining individual electrodes 32 are insulated electrically from eachother by the insulating layer 31.

A wiring 35 (a first wiring) extends from one end (the right end in FIG.2) of each of the individual electrodes 32 in parallel with the longaxis direction thereof on the top surface of the insulating layer 31.The wiring 35 has a terminal 36 (a first terminal) formed on its otherend. All the terminals 36 for the individual electrodes 32 arepositioned at the same height. A driver IC 37 (a drive unit) ispositioned at the top surface of the insulating layer 31 and selectivelysupplies drive voltage to the individual electrodes 32. The driver IC 37has output terminal 37 a, each of which is joined to one of theterminals 36 for the respective electrodes 32 via a bump 38 composed ofconductive soldering material such as solder. In this way, it ispossible to connect the individual electrodes 32 and the driver IC 37directly by the wirings 35, which are formed on the same plane as theseelectrodes, without using a costly wiring member such as an FPC. Thisreduces the cost of electric connection and improves the reliability ofthe connection.

The driver IC 37 also has input terminals 37 b. Connecting terminals 40are formed on the insulating layer 31. Each of the connecting terminals40 is connected to one of the input terminal 37 b of the driver IC 37via a bump 39 formed of solder or the like. This makes it easy toconnect the driver IC 37 and a control unit (not shown) for the driverIC 37 via the connecting terminals 40.

The piezoelectric layer 33 is formed on the top surfaces of theindividual electrodes 32 and is formed mainly of lead zirconate titanate(PZT), which is a solid solution of lead titanate and lead zirconate,and which is a ferroelectric substance. The piezoelectric layer 33 isformed as a continuous layer as to cover the entire surfaces of theindividual electrodes 32. The common electrode 34 is formed on theentire top surface of the piezoelectric layer 33 and is common to theindividual electrodes 32. As is shown in FIG. 2, a wiring 41 (a secondwiring) extends from the common electrode 34 and over the top surfacesof the piezoelectric layer 33 and insulating layer 31. The wiring 41 hasa terminal 42 (a second terminal) formed on its other end, which is incontact with a terminal (not shown) of the driver IC 37. This results inthe common electrode 34 being grounded via the wiring 41 and the driverIC 37 to be kept at ground potential. The common electrode 34 is alsomade of conductive material such as gold. The terminal 42 is positionedat the same height as the terminals 36.

As is shown in FIG. 3, the common electrode 34 and the output terminalsof the driver IC 37 are positioned at different heights because thecommon electrode 34 is formed on the top surface of the piezoelectric33. By contrast, the individual electrodes 32 are connected to thedriver IC 37 by the wirings 35, which are formed on the same plane asthe individual electrodes. As compared with the individual electrodes32, the electric connection of the common electrode 34 and driver IC 37is conceivably difficult to some extent. However, because the commonelectrode 34 is formed to correspond to all of the individual electrodes32 and common to them, the common electrode 34 can be connected to thedriver IC 37 by only one wiring 41. This eliminates the need for using awiring member such as an FPC to connect the common electrode 34 to thedriver IC 37. Because there is only one terminal for the commonelectrode 34, this electrode can be electrically connected easily by aconductive paste or the like, and the connection is highly reliable.

Because the terminals 36 for the individual electrodes 32 and theterminal 42 for the common electrode 34 are formed on the top surface ofthe insulating layer 31, all of these terminals 36 and 42 can bepositioned at the same height. This makes it is easy to join theseterminals 36 and 42 and the output terminals of the driver IC 37, andincreases the reliability of the electric connection of the joinedterminals. When forming all of the terminals 36 and terminal 42 on thetop surface of the insulating layer 31, it is necessary to merely formthe plurality of wirings 35 and one piece of the wiring 41 on thesurface of the insulating layer 31. This makes it possible to positionthe terminals 36 and 42 at the same height with a simple wiringstructure that does not require the use of a complex structure, such asthrough holes.

The wiring 41 extents over a surface having a level difference which isequivalent to the thickness of the piezoelectric layer 33. Therefore, itis considered that the wiring 41 is lower in structural reliability thanthe wirings 35, which extend on a flat surface. However, as describedabove, only one wiring 41, which is relatively unreliable, is enough,and the other wirings 35 are high in structural reliability. Thisresults in an overall reliable wiring structure. If the wiring 41 isformed at one time from the common electrode 34 to the insulating layer31, its portion at the level difference is thinner. Therefore, as shownin FIG. 2, this portion is provided with a reinforcing part 43 forenhancing its reliability.

The action of the piezoelectric actuator 3 during ink discharge will bedescribed below. The driver IC 37 supplies drive voltage selectively toindividual electrodes 32, which are connected there to via therespective wirings 35. As a result, the potential of the individualelectrodes 32 on the bottom side of the piezoelectric layer 33 to whichdrive voltage is applied differs from that of the common electrode 34 onthe top side of the piezoelectric 33, which is kept at ground potential.The potential difference generates an electric field vertically acrossthe piezoelectric layer 33 between the individual electrodes 32 and thecommon electrode 34. The electric field causes the portions of thepiezoelectric layer 33 which are disposed directly above theseindividual electrodes 32 to contract in a horizontal direction. Thehorizontal direction is perpendicular to the vertical direction in whichthe piezoelectric layer 33 is polarized. The insulating layer 31 and thevibration plate 30, which are positioned on the bottom side of thepiezoelectric layer 33, are fixed to the cavity plate 10. Therefore, thehorizontal contraction causes the portions of the piezoelectric layer 33that are sandwiched between these individual electrodes 32 and thecommon electrode 34 to deform so as to project toward the respectivelyassociated pressure chambers 14. The partial deformation of thepiezoelectric layer 33 causes the portions of the vibration plate 30that cover these pressure chambers 14 to deform so as to project towardthe pressure chambers. This reduces the volume of these pressurechambers 14, thereby increasing the pressure on the ink inside. Theincreased pressure discharges ink from the nozzles 20 communicating withthese pressure chambers 14.

A method of manufacturing the piezoelectric actuator 3 will be describedbelow. First, as is shown in FIG. 5, the three plates 10 to 12 ofstainless steel and the vibration plate 30 of stainless steel are joinedtogether by diffused junction or the like. The cavity plate 10 and thevibration plate 30 are both made of the same stainless steel, so theyhave equal coefficients of thermal expansion. Accordingly, there islittle residual stress between the joint surfaces of the cavity plate 10and vibration plate 30, and thus their joining strength is extremelyhigh.

Next, as is shown in FIG. 6, the insulating layer 31 is formed ofalumina, zirconia, or ceramic material such as silicon nitride on thetop surface of the vibration plate 30 (the first step). The insulatinglayer 31 may be formed by an aerosol deposition method for depositingultra-fine particulate material by colliding it onto the surface of thevibration plate at high speed. This method makes it possible to form avery thin layer that is highly insulative even if its thickness is notmore than 5 μm and, in particular, not more than 2 μm. The insulatinglayer 31 may also be formed by a sol-gel method or a sputtering method.

Next, as shown in FIG. 7, the individual electrodes 32, the wirings 35,the terminals 36 and the connecting terminals 40 are formed on the topsurface of the insulating layer 31 (the second step). The individualelectrodes 32, the wirings 35, the terminals 36 and the connectingterminals 40 may be formed at one time by patterning the top surface ofthe insulating layer 31 by screen-printing a conductive paste on thissurface. Alternatively, the top surface of the insulating layer 31 maybe patterned with individual electrodes 32, wirings 35, terminals 36 andconnecting terminals 40 by forming a conductive layer on the entiresurface of the insulating layer 31 by a plating method, a sputteringmethod, a vapor deposition method or another method, and then partiallyremoving the conductive layer by laser processing, a masking and resistmethod or another method.

Next, as shown in FIG. 8, the piezoelectric layer 33 formed of PZT isformed on the top surface of the individual electrodes 32 by an aerosoldeposition method, a sol-gel method, a sputtering method or anothermethod (the third step). Subsequently, the piezoelectric layer 33 isheat-treated at a temperature of approximately 600° C. so that thestructure of the piezoelectric layer 33 is fine. Then, as shown in FIG.9, the common electrode 34, the wiring 41 and the terminal 42 are formedon the entire top surface of the piezoelectric layer 33 by screenprinting, a vapor deposition method, a sputtering method or anothermethod (the fourth step).

Then, as is shown in FIG. 10, the driver IC 37 is placed on theinsulating layer 31. The output terminals 37 a of the IC 37 are joinedto the terminals 36 and 42 via the bumps 38. The input terminals 37 b ofthe IC 37 are joined to the connecting terminals 40 via the bumps 39(the fifth step). Finally, as is shown in FIG. 11, the nozzle plate 13is joined to the bottom surface of the manifold plate 12.

The nozzle plate 13 may be made of metallic material such as stainlesssteel. In this case, the channel unit 2 may be formed in advance byjoining the nozzle plate 13 to the other three plates at the same time.Thereafter, the vibration plate 30, the individual electrodes 32 and thepiezoelectric layer 33 may be laminated in order to the top surface ofthe channel unit 2. Then, the piezoelectric layer 33 may beheat-treated.

The individual electrodes 32, the plurality of terminals 36 for therespective electrodes 32, and the plurality of wirings 35, each of whichconnects the associated electrode 32 and terminal 36, are formed on thetop surface of the insulating layer 31. The driver IC 37, which isjoined to the terminals 36, is placed over the insulating layer 31. Thismakes it possible to connect the individual electrodes 32 and the driverIC 37 directly without having to use a costly wiring member such as anFPC. Consequently, the electric connection is more reliable. Whenforming the individual electrodes 32 on the insulating layer 31 (thesecond step), it is possible to form the individual electrodes 32, theterminals 36, the wirings 35 and the connecting terminals 40 at onetime. This contributes to a shortened manufacturing process.

When forming the common electrode 34 on the top surface of thepiezoelectric layer 33 (the fourth process), it is possible to form thiselectrode 34, the wiring 41 and the terminal 42 at one time. This alsocontributes to a shortened manufacturing process.

Because the common electrode 34 is formed to be common to all of theindividual electrodes 32, only one wiring 41 is required to connect thecommon electrode 34 to the driver IC 37. Thus, there is no need to use awiring member to connect the common electrode 34 to the driver IC 37.Because there is only one terminal for the common electrode 34, it iseasy to connect this electrode electrically, and the reliability of theconnection is high.

Second Embodiment

In the first embodiment, the vibration plate 30 formed of stainlesssteel is used, and the insulating layer 31 of ceramic material isprovided on the top surface of the vibration plate 30. Alternatively,the vibration plate 30 may be formed of insulating material so that theinsulating layer 31 can be omitted. In this case, the vibration plate 30may be formed of alumina, zirconia, silicon nitride or other ceramicmaterial so that the piezoelectric actuator can be manufactured at lowcost. In this case, both of the cavity plate 10 and vibration plate 30may be formed of a same material such as, for example, alumina so thattheir coefficients of thermal expansion are matched. The pressurechambers 14 may be formed by etching the alumina material of the wholechannel unit 2 including the vibration plate 30. The second embodimentis same as the first embodiment except that the vibration plate ischanged and the insulating layer is omitted.

Third Embodiment

In each of the foregoing embodiments, the common electrode 34 is formedon the piezoelectric layer 33 as a rectangular region covering not onlyareas associated with the individual electrodes 32 but also the spacesbetween these areas. However, the common electrode 34 may make any otherpattern as far as the common electrode is common to the individualelectrodes 32. For example, as shown in FIG. 12, the common electrodemay be composed of divisions 34A, each of which is associated with oneof the individual electrodes 32, and which are connected by wirings 34B.The division 34A that is nearest to the reinforcing part 43 may beconnected thereto. The pattern of the connection of the divisions 34Avia the wirings 34B is not limited to that shown in FIG. 12. In thelight of the prevention of capacitance generation, it is preferable thatthe divisions 34A be connected without overlapping vertically with thefirst wirings 35 for the individual electrodes 32. The third embodimentis same as the first embodiment except that the pattern of the commonelectrode is changed.

Modifications to the foregoing embodiments will be described below. Theparts etc. in the modifications that are identical with the counterpartsin the embodiment will be assigned the same reference numerals as thecounterparts. Therefore, detailed descriptions of the modifications willbe omitted.

First Modified Embodiment

In the embodiment described above, the common electrode 34 is formedover all of the individual electrodes 32, which are separated into aplurality of groups. Alternatively, a common electrode 34 may be formedfor each group of individual electrodes 32. Obviously, however, in orderto minimize the number of wirings 41 for connecting the commonelectrodes 34 and the driver IC 37, it is preferable that the groups ofcommon electrodes 34 be small in number.

Second Modified Embodiment

It is not necessary that all three plates 10 to 13 of the channel unit 2be formed of stainless steel. Rather, at least the cavity plate 10,which is laminated with the vibration plate 30 of the piezoelectricactuator 3, may be formed of stainless steel. The material that can beused for the cavity plate 10 is not limited to stainless steel. Rather,the cavity plate 10 may be formed of iron alloy other than stainlesssteel, nickel alloy, aluminum alloy or other material that makes it easyto form the pressure chambers 14 by etching. In order for the vibrationplate 30 and the cavity plate 10 to have similar coefficients of thermalexpansion for better joining between them, it is preferable that thevibration plate 30 be formed of iron alloy, nickel alloy, aluminum alloyor the like, as is the case with the cavity plate 10. It is particularlypreferable that the vibration plate 30 and the cavity plate 10 be formedof the same material.

Third Modified Embodiment

The method for manufacturing the piezoelectric actuator 3 of theforegoing embodiment includes the step of heat-treating thepiezoelectric layer 33 after forming this layer by an aerosol depositionmethod, a sol-gel method, a sputtering method or another method.Alternatively, the piezoelectric layer 33 can be formed by bonding apiezoelectric sheet composed of calcinated PZT to the insulating layer31. In this case, a common electrode 34 is formed in advance on the topsurface of the piezoelectric sheet by screen printing or the like. Then,the bottom surface of the piezoelectric sheet is joined to the topsurfaces of the individual electrodes 32. Alternatively, a green sheetof PZT that can be calcinated at low temperature may be formed on thetop surfaces of the individual electrodes 32 by screen printing or thelike. In this case, the green sheet needs to be calcinated at atemperature between 850 and 900° C. at a later step.

Fourth Modified Embodiment

The driver IC 37 does not necessarily need to be positioned over theinsulating layer 31. Rather, it is essential that at least the terminals36, to which the output terminals 37 a of the driver IC 37 are joined,be formed on the insulating layer 31.

FIG. 13 shows a preferred embodiment of the ink-jet recording printerprovided with the ink-jet head of the present invention. The ink-jetrecording printer 100 shown in FIG. 13 includes four ink cartridges 61which include respective colors of, for example, black, yellow, magenta,and cyan inks respectively, a head unit 63 which includes a multi-head 6for discharging ink droplets onto printing paper 62, a carriage 64 onwhich the ink cartridges 61 and the head unit 63 are provided, a driveunit 65 which reciprocates the carriage 64 in the linear direction, aplaten roller 66 which extends in the direction of the reciprocatingmotion of the carriage 64 and which is arranged opposingly to themulti-head 6, and a purge unit 67. The multi-head 6 includes an ink-jethead 1 of the above-described embodiment for each of the colors.

The drive unit 65 includes a carriage shaft 71, a guide plate 72, twopulleys 73, 74, and an endless belt 75. In this arrangement, thecarriage shaft 71 and the guide plate 72 extend in parallel to theplaten roller 66 to slidably support the carriage 64. The endless belt75 is stretched between the pulleys 73, 74′. When the pulley 73 isrotated in the positive direction and the opposite direction, thecarriage 64, which is connected to the endless belt 75, makes thereciprocating motion in the linear direction along the carriage shaft 71and the guide plate 72 in accordance with the positive rotation and theopposite rotation of the pulley 73.

The printing paper 62 is supplied from a printing paper cassette (notshown) which is connected to the ink-jet recording printer 100. Theprinting paper 62 is fed into the space between the multi-head 6 and theplaten roller 66. The programmed printing is carried out thereon withthe ink droplets discharged from the multi-head 6. After that, theprinting paper 62 is discharged to the outside. The printing papersupply function and the printing paper discharge function are not shownin FIG. 13.

The purge unit 67 is installed on one side of the platen roller 66. Thepurge unit 67 includes a purge cap 81, a pump 82, a cam 83, and a drainink reservoir 84. When the head unit 63 is placed at the positionopposed to the purge cap 81, then the purge cap 81 is moved upwardly bythe cam 83 to make contact with the nozzle surface of the multi-head 6,and a plurality of nozzles formed in the multi-head 6 are coveredtherewith. The ink contained in the multi-head 6 is sucked by the pump82, and thus any ink having the increased viscosity and bubbles aredischarged from the inside of the multi-head 6. The sucked ink is storedin the drain ink reservoir 84. The cap 85 is installed in order toprevent the ink from being dried. When the carriage 64 is returned tothe reset position after the printing, the cap 85 covers the nozzles(not shown) of the multi-head 6 provided on the carriage 64.

1. A piezoelectric actuator comprising: a metallic vibration plate; aninsulating layer formed on a surface of the vibration plate; a pluralityof individual electrodes formed on a surface of the insulating layer; apiezoelectric layer formed on surfaces of the individual electrodes; acommon electrode formed on a surface of the piezoelectric layer andcommon to the individual electrodes; a plurality of first terminalsformed on the surface of the insulating layer and each associated withone of the individual electrodes; and a plurality of first wiring formedon the surface of the insulating layer and each electrically connectingone of the individual electrodes to one of the first terminals.
 2. Thepiezoelectric actuator according to claim 1, further comprising: asecond terminal formed on the surface of the insulating layer andassociated with the common electrode; and a second wiring formed on thesurface of the insulating layer and electrically connecting the commonelectrode and the second terminal.
 3. The piezoelectric actuatoraccording to claim 1, wherein the insulating layer is formed of aceramic material.
 4. The piezoelectric actuator according to claim 2,further comprising a drive unit which supplies drive voltage selectivelyto the individual electrodes, the drive unit being placed on the surfaceof the insulating layer, the drive unit having a plurality of outputterminals joined to the first terminals and the second terminal.
 5. Thepiezoelectric actuator according to claim 4, wherein the drive unitfurther has an input terminal, the piezoelectric actuator furthercomprising a connecting terminal which electrically connects the driveunit and a control unit which controls the drive unit, the connectingterminal being formed on the surface of the insulating layer, theconnecting terminal being joined to the input terminal of the driveunit.
 6. The piezoelectric actuator according to claim 1, wherein theinsulating layer has a thickness of not more than 5 μm.
 7. Thepiezoelectric actuator according to claim 6, wherein the insulatinglayer has a thickness of not more than 2 μm.
 8. The piezoelectricactuator according to claim 1, wherein the common electrode is grounded.9. An ink-jet head comprising: the piezoelectric actuator as defined inclaim 1; and a channel unit having a plurality of pressure chambers andnozzles which discharge ink, the pressure chambers each communicatingwith one of the nozzles, the pressure chambers being open on a surfaceof the channel unit, and pressure chambers each being associated withone of the individual electrodes of the piezoelectric actuator; whereinthe vibration plate of the piezoelectric actuator is formed of any oneof iron alloy, nickel alloy and aluminum alloy, and laid on the surfaceof a portion of the channel unit; and wherein at least the portion ofthe channel unit on which the vibration plate is laid is formed of anyone of iron alloy, nickel alloy and aluminum alloy.
 10. The ink-jet headaccording to claim 9, wherein the vibration plate is formed of a samematerial as that of at least the portion of the channel unit on whichthe vibration plate is laid.
 11. The ink-jet head according to claim 10,wherein the vibration plate and at least the portion of the channel uniton which the vibration plate is laid are formed of stainless steel. 12.An ink-jet printer provided with the ink-jet head as defined in claim 9.13. A method for manufacturing a piezoelectric actuator, the methodcomprising: a first step of forming an insulating layer on a surface ofa metallic vibration plate; a second step of forming a plurality ofindividual electrodes, first terminals and first wirings on a surface ofthe insulating layer, the first terminals each being associated with oneof the individual electrodes, the first wirings each electricallyconnecting one of the individual electrodes and one of the firstterminals; a third step of forming a piezoelectric layer on surfaces ofthe individual electrodes; and a fourth step of forming a commonelectrode on a surface of the piezoelectric layer over the individualelectrodes.
 14. The method according to claim 13, wherein the fourthstep includes forming a second terminal and a second wiring togetherwith the common electrode on the surface of the insulating layer, thesecond terminal being associated with the common electrode, the secondwiring electrically connecting the common electrode and the secondterminal.
 15. The method according to claim 13, further comprising afifth step of: placing, on the surface of the insulating layer, a driveunit which supplies drive voltage selectively to the individualelectrodes, and has output terminals; and joining the output terminalsto the first terminals and the second terminal.
 16. The method accordingto claim 15, wherein the second step further includes forming, on thesurface of the insulating layer, a connecting terminal whichelectrically connects the drive unit and a control unit which controlsthe drive unit, the connecting terminal being joined to an inputterminal of the drive unit.
 17. The method according to claim 14,wherein the insulating layer is formed on the surface of the vibrationplate by an aerosol deposition method in the first step.
 18. Apiezoelectric actuator comprising: a piezoelectric layer; a vibrationplate provided on one surface of the piezoelectric layer; a plurality ofindividual electrodes interposed between the piezoelectric layer and thevibration plate; a common electrode provided on the other surface of thepiezoelectric layer and common to the individual electrodes; a pluralityof first terminals formed on a same plane as the individual electrodesand each associated with one of the individual electrodes; and aplurality of first wirings formed on the same plane as the individualelectrodes and each electrically connecting one of the individualelectrodes and one of the first terminals.
 19. The piezoelectricactuator according to claim 18, wherein the vibration plate is formed ofmetallic material, the piezoelectric actuator further comprising aninsulating layer formed on the vibration plate.
 20. The piezoelectricactuator according to claim 18, wherein the vibration plate is formed ofinsulating material.
 21. The piezoelectric actuator according to claim18, further comprising: a second terminal formed on the same plane asthe individual electrodes and associated with the common electrode; anda second wiring formed on the same plane as the individual electrodesand electrically connecting the common electrode and the secondterminal.
 22. The piezoelectric actuator according to claim 18, wherein:the plurality of individual electrodes, the first terminals, the secondterminal, the first wirings and the second wiring are formed on asurface of the insulating layer; and a drive unit which supplies drivevoltage selectively to the individual electrodes is provided on thesurface of the insulating layer, the drive unit having a plurality ofoutput terminals joined to the first terminals and the second terminal.23. The piezoelectric actuator according to claim 18, wherein the commonelectrode is grounded.
 24. An ink-jet head comprising: the piezoelectricactuator as defined in claim 18; and a channel unit having a pluralityof pressure chambers and nozzles which discharge ink, the pressurechambers each communicating with one of the nozzles, the pressurechambers being open on a surface of the channel unit, the pressurechambers each being associated with one of the individual electrodes ofthe piezoelectric actuator.